How do you find the Heat Zone for specific plants ? I have fallen in love with many of the plants shown on this site and know many of them can survive quite well in my cold zone of 8a. However, I am finding it difficult if they can thrive in my Heat zone of 8.

This is a quote from the AHS website about heat zones and their heat zone map:

"The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days)."

It's great to know so many plants can survive my winter cold temps, but the idea of having plants that survive the winter and then end up looking fried because they cannot survive my high summer temps leaves me wondering if a given plant should be planted in this garden.

Smiles,
Lyn

Short answer: I don't know and could not find actual numbers for specific species. maybe other people who live in hot-summer climates (like Texas) can suggest some genera to avoid, or to try.



http://www.degreedays.net/ (another degree-day calculator)
http://whyfiles.org/2010/what-...

I would be really interested if you find that "stress temperature" for different species, or even a "low temperature" at which little development occurs. I couldn't. So far my best Google search term was "plant heat stress".

One source admitted that even the lower growth thresholds of most plants are not known (coldest temp at which a plant speices grows or matures)
http://www.oardc.ohio-state.ed...

This source describes the lower and upper limuts well, but doesn't give actual tempertaures for any plant but corn:
http://ndawn.ndsu.nodak.edu/he...

"... above a threshold or base temperature below which little growth occurs. This lower threshold temperature varies with plant species. Scientists have determined the lower base temperature for corn is 50 °F (10 °C). In some plants there is also an upper threshold temperature, above which conditions are stressful to the plants. The upper limit for corn is 86 °F (30 °C). "

Here's a PDf that goes into some detail:
http://ucanr.edu/sites/MarinMG...
"Heat stress is often defined as a period in which temperatures are hot enough for a sufficient period of time to cause irreversible damage to plant function or development. Plants can be damaged by either high day or high night temperatures and by either high air or soil temperatures."


My guess is that the upper "stress threshold" is particulalry dependent upon water availability to the plant, humidity, wind, clouds, and degree of shade or exposure.

Since the high-temperature effect is "stress", it might not be tracked very well by the daily average temperature accumulated heat units. The average of the day's lowest temperature and highest temperature might never go near the stress threshold if nights cool down even briefly. And yet the temeprature might exceed the stress point for several hours.

Guessing even further, maybe a better statistic for tracking plant's stress level due to high temperatures would be "degree hours" or 'degree minutes" - the total number of hours or minutes when the average temperature for that minute exceeded the threshold.

Yet another guess: the temperature that stresses the plant would vary, depending on the age of the plant and perhaps the size of the root system relative to the foliage..

RickCorey said:My guess is that the upper "stress threshold" is particulalry dependent upon water availability to the plant, humidity, wind, clouds, and degree of shade or exposure.

Guessing even further, maybe a better statistic for tracking plant's stress level due to high temperatures would be "degree hours" or 'degree minutes" - the total number of hours or minutes when the average temperature for that minute exceeded the threshold.

Yet another guess: the temperature that stresses the plant would vary, depending on the age of the plant and perhaps the size of the root system relative to the foliage..

The validity, suppositions and assumptions of these are really fodder for a separate thread, don't at all answer Rosebush1's question, and steers the discussion away form its intended course. Maybe start a new topic in a different forum. I think we should really try hard to respect Dave's wishes, as he articulates in big green lettering above every Reply message box in this Questions Forum.

Don't a lot of plants in the database specify both a minimum zone and a maximum zone for growing? Or is that even what is being asked? I've noticed that sometimes individual plant entries don't have a lot of specific information, but if you look at the parent entry, you can get a better idea of what they'll tolerate.

I think that the zones recommended for a plant are based on its cold tolerance not its heat resistance. Do the sunset zones help?

Rose,

I don't know much about it personally but based on what I've read posted on various blogs and gardening websites, many Californian gardeners swear by the 'Sunset Zones' because they take into consideration many of the factors that create the large variety of micro-climates in their state. Below are a couple of links to check out.

Sunset zone info:
http://www.sunset.com/garden/c...

Plant Finder with Sunset zone finder:
http://plantfinder.sunset.com/...

I definitely understand the importance of knowing the heat tolerance of plants since we have very hot temperatures here too. It's so disappointing to see a new plant that was gorgeous in spring suddenly melt into a moldy puddle when the summer temperatures arrive. Hopefully someone from California will check in here soon and give you some specialized advice.

Rick .... I've searched for this kind of information for a while and it's good to know that others find it as difficult to find specific information, too.

The only genus of plants that I know well happens to be roses ... a woody plant. Knowing the botany of the plant has helped me select roses that do quite well in my hot summer temps. It's not the availability of water that creates heat stress, but how the plant moves moisture through the plant. A rose can be well watered, but if the transpiration rate is higher than the plant can pull moisture up to the top growth, the plant will experience heat stress and will start discarding growth it cannot support. Many roses go dormant in periods of high temps. Often a rose will abandon leaves it cannot support, because it is in survival mode. Over time, I've learned what plant characteristics a rose must have to thrive in my climate, but now I want to expand my plant pallet and had hoped others would know more about heat zones than I do.

Porkpal ... no the Sunset zones do not help. They, too are geared towards cold tolerance. I am in Sunset zone 7. My summer temps are generally in the 90s to low 100s and I believe the heat stress is a far greater issue for selecting plants than cold hardiness. Other than tropicals, it seems that there is a wide range of plants that can handle my cold temps during the winter.

Danita ....... Thanks for the link, I think it's easier to use than the book. California has all 24 Sunset zones and all 11 AHS cold hardiness zones.

I have no shade in my gardening area ... the area protected from deer ... and my full sun in Heat zone 8 is very different from full sun in Heat zone 4 or 5 like Rick has in Washington.

Since most nurseries don't supply heat zone information, I'll have to ask about specific cultivars on the various forum.

Thank you all for taking the time to reply.

Smiles,
Lyn

>> A rose can be well watered, but if the transpiration rate is higher than the plant can pull moisture up to the top growth,

I think you're completely right. Plus, plain old tissue temperatures might prevent blooms - I've read that tomatoes stop blooming or setting fruit around (??) 90 or 95 F (??).

Maybe a local cooperative extension agency or University Ag department has lists of species that work locally. I find a lot of "local gardening advice" there.

I see I need to update the link in my sig block!

WAS:
http://www.csrees.usda.gov/ind...

IS:
http://www.nifa.usda.gov/Exten...
http://cagardenweb.ucdavis.edu...

They at least might point you towards another place to ask detailed questions.

Hi Rick .......

>>>>> A rose can be well watered, but if the transpiration rate is higher than the plant can pull moisture up to the top growth,

I think you're completely right. Plus, plain old tissue temperatures might prevent blooms - I've read that tomatoes stop blooming or setting fruit around (??) 90 or 95 F (??).

Yes, you are right about tissue structure in a rose can impact its heat tolerance. I have found that roses with thick petal substance and dense foliage are the most successful roses in my heat zone. That leaves out a lot of different classes of roses, but fried blooms are not all that attractive.

I am certain that must be true for a lot of other types of plants.

I know from experience that the fruit set of tomatoes is impacted by high temps. If my toms have not blossomed before the heat hits, I end up with a low yield for the season.

I do get a lot of general information from the UCDavis site and know several of the people in their horticultural department ... funny how we all end up meeting people through our gardening hobby ... that have been quite helpful. The problem comes down to finding the right cultivars that will do well at a higher elevation with more intense light and high heat.

My night temps can swing down as much as 50 degrees which gives me an advantage in that the plants have time to rehydrate, while UC Davis is located down in the valley where the night temps don't cool off, so we are always guessing about what will work.

Thanks for the links. I'll check them out tonight. I am off to go collect mulch before it rains.

Smiles,
Lyn

>> The problem comes down to finding the right cultivars

Ouch! That's pretty detailed. Good luck!

Rick...

People have been gardening all over the world without taking heat zones into consideration for plant selection for centuries. The concept of heat zones is relatively new when compared to thinking in terms of cold hardiness zones. I think that's why the information is not readily available.

However, with the links provided in this thread, I do have some new starting points for doing some research. Thanks to all of you for helping me along my way.

Smiles,
Lyn

This thread is the reason forums are so very helpful. I started gardening around the time when every magazine had "The English Border" garden on their covers. The advice regarding cold tolerance was spot on. When the perennials popped out of my spring soils, it was exactly as promised. Vibrant lovely plants left me feeling like a real gardener, not the novice I was. August temps, however brought me back down to earth, as I was staking and watering and removing plants that were ready for fall to come back, right when I had about 3 weeks of hot sun left for them to endure.

I will be thinking of this very good question every time I am considering a new plant for the yard.

Hi Laurie ...

I've been meaning to respond to your post for a while, but by the time I sit down to the computer in the evening, my brain seems to be a dull tool ...

I've had several more years experience gardening in this climate since my initial post about this topic and have had a chance to observe how much more important it is for me to know about heat zone information for a plant, or more specifically a cultivar, is before I buy it than I understood years ago.

It's not just about fried blooms, but about the survival of the plant. It's amazing how cold hardiness information is readily available on plant tags at nurseries, but I have yet to see heat zone information on plant tags. It makes all the difference in m garden.

I agree that heat tolerance needs to be rated not just cold hardiness. I frequently admire old Galicas but know they seldom do well here. The label nor the nursery's description, however, give no hint. Fortunately Melva in central Texas has been able to grow a few and I have learned from her experience.

Porkpal ...

With Galicas, it's a different issue than heat zones ... Galica roses need chill hours. Heat zones are more about heat tolerance, which is another aspect of heat impacting plant performance.

Hi @RoseBush1:

Do you ever have nights when something keeps you from sleeping, maybe a little gardening injury, or some little ache and pain, that may or may not be due to age. And one sleepless night becomes 3 or 4. Basically, I should hide the computer from myself those nights, I totally forgot to check the date of this thread. Doh.

I am sorry to open up such an old thread.

But, it's on you just a little, because it was a very intreguing question!!

Sorry, you were very kind about it.

Laurie B

No problem, Laurie

The sad part is that there still is the information about heat zones is still not readily available.

I understand the "why" of it a lot better and know that it would be very difficult to provide that information to consumers.

When heat zones are assigned, they are assigned by the number of days that temperatures exceed 85F, with no consideration as to humidity in a given climate. Humidity plays a huge role in the loss of moisture through the leaves of a plant due to high temperatures. So, just knowing the heat zone is not enough information to make a good decision about whether or not a plant will do well in your climate or where to site a plant in your garden.

Gardening can be kind of tricky that way.

Edited to add:

Hope you are feeling better soon ... get some sleep !

lauriebasler said: ...
Do you ever have nights when something keeps you from sleeping, maybe a little gardening injury, or some little ache and pain, that may or may not be due to age. And one sleepless night becomes 3 or 4. ...

Yup, "too tired to sleep". It feels like going crazy. I used to work a night sift in a chemical factory where we had two 12 hour work days, plus three 6-hour work days, unless they happened to become 5, 12-hour work days.

Miss one day of sleep, and turn into a zombie until the next 2 days off. Like jet lag but without the travel.

Honestly, i don't make much of them, but then I live in Australia where there'll be several days of 40C temps every year and around half the days between October through april is above 30C.

The only issues I have with plant viability is when they aren't established, if the root system hasn't developed well (like after I've transferred seedlings) and then a few days of very hot days they get fried (but that's not surprising, inability to access water tends to result in death for anything) other than that though no issues. No sudden death, no irreparable damage (some plants wilt a but but bounce back over night).

Not really like cold tolerance (which generally persists for weeks at a time rather than every other day) with death very likely, so that probably plays a part in the lack of info on the topic.

@Protoavis

It makes sense that you would not be very concerned given the situation you have described above. ...

In my arid summer climate, the summer day temps are between 35 to 40C every day for about 4 months. That is hard on even established plants, unless you select plants that can survive in that kind of heat with low humidity.

I am lucky in that I live in the mountains where I have a 40 to 50F degree temperature change (4.5 - 10C) between day and night temps that allows the plants to rehydrate. However, this year was different. The temps did drop, but only for about two hours and the plants did not have sufficient time to rehydrate and were quite heat stressed.

A plant can have sufficient moisture in the root zone, but if it cannot pull moisture up to the top growth, the leaves cannot perform the function of photosynthesis and thus the plant is heat stressed to the point where survival becomes an issue.

You are correct in that it is vital that one of the best techniques for gardening in this kind of climate is to make sure that any plant has a larger root mass before it is placed in the ground. Young plants simply do not have the tools (roots) to survive in this kind of heat.